The submitted invention concerns a fitting, especially as applied in a continuous flow water heater to control gas flow depending on water flow. Thereby the fitting has a gas control housing, a water control housing and a mechanical transmission which transmits a control motion, depending on the water flow and arising preferably from a Venturi tube and a membrane in the water control housing, extending initially from the interior of the water control housing, then outwards to a valve controlling the volume of gas within the gas control housing.
Fittings for continuous flow water heaters usually consist of gas and water components. In these fittings, gas control is activated by a hydraulic adjusting drive, which produces a particular stroke to control the volume of gas depending on the volume of water flowing through the fitting. Thereby the stroke starts mostly at particular minimal value, determined by the necessary volume of gas required to ignite the burner, and finishes at a particular maximum value which is determined by the technical layout of the gas burner and the heat exchanger.
As already discussed above and, for example, as shown in DR-PS 647 449, a control device, located in the water component, has a membrane through which the control housing is divided into chambers which are connected via pipes with parts of the fluid pipe which lies before and behind a pressure regulating valve. A gas pressure switch, located in the gas component, has a valve with a valve body which is moved by a further member. The membrane is controlled by pressure difference produced by a pressure regulating valve in a gas pipe leading to the burner. The control device is connected via a rod with a valve body belonging to the pressure regulating valve.
Another fitting for a continuous flow water heater is described in WO 94/00716. In this case, a control device, designated as a flow switch, is located in the water component and serves as a water deficiency safety device. This flow switch has a high pressure chamber and a low pressure chamber which are separated from each other by a membrane. The membrane is supported on a membrane plate rigidly connected with a rod which itself is sealed and leads out of the low pressure chamber. Thereby the membrane, membrane plate and rod form the adjusting drive for a gas volume control valve arranged in the gas component.
The construction of the fittings described in the above named patent documents have a problem in that the rod, serving to transmit the stroke, carries out an axial motion. Due to this axial motion, the rod transports water particles despite a seal being present. In order to prevent water particles getting into the gas control space, it is usually necessary that there be an air space between the water and gas control spaces. Additionally, the rod is made in two pieces. The problem still remains that by increasing rod stroke due to increasing water usage, a film of water on the rod is brought outside by the axial motion and assisted by the water pressure. This film of water evaporates in the air and leads to calcareous deposits on the rod. During the backstroke of the rod, due to decreasing water usage, this calcareous layer is drawn through the seal which damages the seal and thus leads to greater leakage. Furthermore, the control deviation of the continuous flow water heater is enlarged due to the increasing friction of the rod. Especially by the reduction of the volume of water flowing through, the volume of gas is no longer sufficiently reduced which leads to water temperatures being too high and the corresponding hazards to the user arise. The calcareous deposits can extend so far that the gas volume control valve remains open when the water supply is completely cut-off, which leads to destruction of the continuous flow water heater if special over-temperature protection device is not present.
By the employment of grease seals, double gaskets and ceramic guides, one can indeed delay these negative consequences but water transport, due the axial motion of the rod, remains thus leading to a breakdown at some time or other. Furthermore, such measures have the disadvantage that the constructional size and finally the costs are increased.
The invention takes as its basis the problem of developing a fitting of the above mentioned type by which the formation of calcareous deposits on the rod protruding from the water control housing is lessened and the influence of such calcareous deposits on the controller action, as described in detail above, is completely eliminated.
According to the invention, the problem is solved in that within the water control housing, the control motion of the membrane is transmitted to a lever whose centre of rotation forms a shaft, rigidly connected to this lever and whose pivot is sealed, leading to the exterior of the water control housing. At the same time, the shaft is also the centre of rotation for a second lever which is likewise rigidly connected to it. Thus the stroke motion of the first lever is transmitted to the exterior via the shaft as a rotary motion and subsequently the rotary motion of the shaft is again converted, by means of the second lever, into a stroke motion to control the gas outlet.
By converting the stroke motion of the membrane to a rotary motion, the situation is achieved whereby the sealing element required for sealing, usually an O-ring, always seals the shaft at the same position. An axial motion no longer occurs in the sealing area. Thus the previously occurring transport of water particles from the water control housing and the thereby associated negative consequences do not take place. Even if a leakage flux should occur, despite the seal found between the water control housing and the shaft, and calcareous deposits forms on the outer end of the shaft, this has no influence on bearing friction and controller action. As a further advantage, the influence of water pressure on the volume of gas flow, as described in the state-of-the-art, (the cause being due to the effective force components of water pressure on the cross-section of the rod) is not encountered with the solution, offered by the present invention.
Through the measures laid out in the sub-claims, advantageous further developments of the object of the main claim are possible.
Thus the fitting, according to the invention, is constructed such that the second lever outside the water control housing is connected to the shaft in one piece. The resulting stroke motion of the second lever is then transmitted by the known method via a rod in the gas control housing to a valve controlling the volume of gas. Thereby, a connecting rod element can be arranged additionally between the second lever and the rod, whereby at the same time an adjustment position is created.
A further advantageous form of construction of the invention consists in that the shaft, whose pivot is sealed, leads into the interior of the gas control housing, in which the second lever is arranged such that the transmission of the control motion is likewise no longer axial. It is more favourable if the shaft is made in two parts, whereby a first part of the shaft leads from the interior of the water control housing to its exterior whereas a second part of the shaft leads from the interior of the gas control housing to its exterior. Thereby it is made possible to create an adjustment position at the transition between the first and second parts of the shaft.